In one form of known variable flow rate control valve, the valve body defines a valve chamber having an inlet port and an outlet port communicating therewith. Means are provided for connecting the inlet port to a source of pressurized fluid and a valve member is provided in the valve chamber for selectively controlling the rate of flow of the pressurized fluid from the inlet port to the outlet port.
Means are provided for urging the valve member from a closed position closing the inlet port to an open position, permitting communication between the inlet port and the valve chamber to permit fluid flow through the valve. A common means for positioning the valve comprises an electrical solenoid operator which acts in opposition to spring means which biases the valve member to the closed position. The force produced by the solenoid operator, when energized, and the spring force opposing the solenoid operator are preselectedly coordinated to cause the valve member to be positioned in a preselected open position when the solenoid operator is energized. This provides a preselected amount of opening of the inlet port to provide substantially the desired flow rate of fluid through the valve as a result of the fluid pressure differential thereacross and the amount of the opening of the inlet port.
A problem arises in such valves in that the pressure differential across the valve may change due to a change in the load pressure characteristics. This causes a change in the flow rate which is proportional to the square root of the pressure change.
Thus, while accurate positioning of the valve member may be effected in such valves, this does not assure corresponding accurate maintained fluid flow rate.